In the past few decades, plant scientists have begun to realize that plants may be serving as a reservoir of untold numbers of organisms known as endophytes (Bacon, C. W., and White, J. F. 2000. Microbial Endophytes. Marcel Deker Inc., N.Y.). By definition, these microorganisms (mostly fungi and bacteria) live in the intercellular spaces of plant tissues. Some of these endophytes may be producing bioactive substances that, in some way, may be involved in the host-endophyte relationship. As a direct result of the role that these secondary metabolites may play in nature, they may ultimately be shown to have applicability in medicine, agriculture and industry. We are now witnessing the beginning a worldwide scientific effort to isolate endophytes and study their natural products. While there are myriads of epiphytic microorganisms associated with plants, the endophytic ones seem to be attracting more attention. This may be the case since closer biological associations may have developed between these organisms in their respective hosts than the epiphytes. Hence, the result of this may be the production of a greater number and diversity of classes of biological derived molecules possessing a range of biological activities. In fact, a recent comprehensive study has indicated that 51% of biologically active substances isolated from endophytic fungi were previously unknown (Schutz, B. 2001. British Mycological Society, International Symposium Proceedings, Bioactive Fungal Metabolites—Impact and Exploitation. University of Wales, Apr.). This compares with only 38% novel substances from soil microflora.
One of the least studied biochemical-chemical systems in nature is the relationship existing between microorganisms and their plant hosts. For instance, it does appear that all higher plants are hosts to one or more endophytic microbes. These microbes include the fungi, bacteria and actinomycetes and reside in the tissues beneath the epidermal cell layers. It is well understood that endophytic infections are at least inconspicuous. And as a result, the host tissues are transiently symptomless and the colonization of the tissues is internal to the surface of the plant. The exact physical relationship of the endophyte to the plant has, in most cases, remained obscure because it is extremely difficult, by electron microscopic techniques, to find an endophyte within plant tissues. Conceivably, the microbes live within the intercellular spaces of the tissues and it also seems likely that penetration of living cells may occur but is not easy to observe.
Fossil records indicate that fungi have been associated with plants since at least 400 million years ago (Simon, et al., Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants, Nature 363, 67-69 (1993); Remy, et al., Four hundred-million-year-old vesicular arbuscular mycorrhizae, Proc. Nat. Acad. Sci. 91, 11841-11843 (1994); Redecker, et al., Fungi from the Ordovician, Science 289, 1920-1921 (2000), all of which are expressly incorporated by reference) and it is theorized that early symbiotic interactions were responsible for the establishment of land plants ( Pirozynski, et al., The origin of land plants: a matter of mycotrophism, Biosystems 6, 153-164 (1975), expressly incorporated by reference). Since the first description of plant/fungal symbiosis (De Bary, A., Die Erschenung Symbiose, in Vortrag auf der Versammlung der Naturforscher and Artze zu Cassel (ed. Trubner, K. J.) 1-30 (Strassburg, 1879); Hertig, M., et al., The terms symbiosis, symbiont and symbiote, J. Parasit. 23, 326-329 (1937) all of which are expressly incorporated by reference), all plants studied in natural ecosystems have been found to be symbiotic with fungi ( Petrini, O., Taxonomy of endophytic fungi of aerial plant tissues, in Microbiology of the Phyllosphere (eds. Fokkema, N. J. & van den Heuvel, J.) 175-187 (Cambridge University Press, Cambridge, 1986) expressly incorporated by reference). These fungi, termed endophytes, express a variety of symbiotic lifestyles including mutualism, commensalism, or parasitism that positively, neutrally, or negatively affect host fitness, respectively (Lewis, D. H., Symbiosis and mutualism: crisp concepts and soggy semantics, in The Biology of Mutualism (ed. Boucher, D. H.) 29-39, (Croom Helm Ltd, London, 1985), expressly incorporated by reference).
The host range, here defined as the ability to colonize a plant, of most symbiotic fungi is poorly defined. With the exception of vesicular arbuscular mycorrhizae, there are few reports of fungal symbionts asymptomatically colonizing both monocots and eudicots (Smith, A. F. & Smith, S. E., Structural diversity in (vesicular)-arbuscular mycorrhizal symbioses, New Phytol. 137, 373-388 (1997); Jumpponen, A. & Trappe, J. M., Dark septate endophytes: a review of facultative biotrophic root-colonizing fungi, New Phytol. 140, 295-310 (1998); Bordallo, J. J. et al., Colonization of plant roots by egg-parasitic and nematode-trapping fungi, New Phytol. 154, 491-499 (2002) all of which are expressly incorporated by reference). This may indicate host range limitations or a limited number of plant taxa analyzed during host range studies. Moreover, individual fungi can express different lifestyles in different plant hosts and, although the basis of symbiotic communication responsible for the outcome of these associations (mutualistic, commensal, or parasitic) is unknown, lifestyle expression appears to be controlled by the plant genome ( Smith, K. P. & Goodman, R. M., Host variation for interactions with beneficial plant-associated microbes, Annu. Rev. Phytopathol. 37, 473-492 (1999); Redman, et al., Fungal symbiosis: from mutualism to parasitism, who controls the outcome, host or invader? New Phytol. 151, 705-716 (2001) all of which are expressly incorporated by reference).
Adaptation of plants to selective pressures is also considered to be regulated by the plant genome (Smallwood, M. F., Calvert, C. M. & Bowles, D. J. Plant Responses to Environmental Stress (BIOS Scientific Publishers Limited, Oxford, 1999) expressly incorporated by reference). However, recent studies indicate that fitness benefits conferred by mutualistic fungi contribute to plant adaptation (Clay, K. & Hotah, J., Fungal endophyte symbiosis and plant diversity in successional fields, Science 285, 1742-1744 (1999); Morton, J. B., Biodiversity and evolution in mycorrhizae in the desert, in Microbial Endophytes (eds. Bacon, C. W. & White, J. F. J.) 3-30 (Marcel Dekker, Inc., New York, N.Y., 2000); Redman, et al., Thermotolerance conferred to plant host and fungal endophyte during mutualistic symbiosis. Science In Press (2003) all of which are expressly incorporated by reference). Mutualistic fungi may confer tolerance to drought (Bacon, C. W., Abiotic stress tolerances (moisture, nutrients) and photosynthesis in endophyte-infected tall fescue, Agricult. Ecosys. Environ. 44, 123-141 (1993); Read, D. J., Mycorrhiza—the state of the art, in Mycorrhiza (eds. Varma, A. & Hock, B.) 3-34 (Springer-Verlag, Berlin, 1999) all of which are expressly incorporated by reference), metals ( Read, D. J., Mycorrhiza—the state of the art, in Mycorrhiza (eds. Varma, A. & Hock, B.) 3-34 (Springer-Verlag, Berlin, 1999) all of which are expressly incorporated by reference), disease (Carroll, G. C., The biology of endophytism in plants with particular reference to woody perennials, in Microbiology of the Phyllosphere (eds. Fokkema, N. J. & Van Den Heuvel, J.) 205-222 (Cambridge University Press, Cambridge, 1986); Freeman, S. & Rodriguez, R. J., Genetic conversion of a fungal plant pathogen to a nonpathogenic, endophytic mutualist. Science 260, 75-78 (1993); Redman, et al. Biochemical analysis of plant protection afforded by a nonpathogenic endophytic mutant of Colletotrichum magna. Plant Physiol. 119, 795-804 (1999) all of which are expressly incorporated by reference), and herbivory (Latch, G. C. M. Physiological interactions of endophytic fungi and their hosts, Biotic stress tolerance imparted to grasses by endophytes, Agricult. Ecosys. Environ. 44, 143-156 (1993) expressly incorporated by reference), and/or promote growth (Marks, S. & Clay, K., Effects of CO2 enrichment, nutrient addition, and fungal endophyte-infection on the growth of two grasses, Oecologia 84, 207-214 (1990); Varma, A. et al., Pirifmospora indica, a cultivable plant-growth-promoting root endophyte, App. Environ. Microbiol. 65, 2741-2744 (1999); Redman, R. S. et al., Field performance of cucurbit and tomato plants colonized with a nonpathogenic mutant of Colletotrichum magna (teleomorph: Glomerella magna; Jenkins and Winstead), Symbiosis 32, 55-70 (2002) all of which are expressly incorporated by reference) and nutrient acquisition (Read, D. J., Mycorrhiza—the state of the art, in Mycorrhiza (eds. Varma, A. & Hock, B.) 3-34 (Springer-Verlag, Berlin, 1999) expressly incorporated by reference).
However, microbes have not been cultured from plants in geothermal soils or reported to contribute to the survival of these plants.
Accordingly, there is a need for compositions and methods to treat plants, including both monocots and dicots, to confer stress tolerance, particularly thermal and drought tolerance.